GAZA CITY (CNN) --Palestinian militant groups Thursday declared an end to their temporary cease-fire with Israel shortly after a senior Hamas leader was killed in an Israeli missile attack in Gaza.

Hamas, Islamic Jihad and the Al Aqsa Martyrs Brigades -- the militant offshoot of Palestinian Authority President Yasser Arafat's Fatah movement -- said they would no longer abide by the cease-fire.

Israel contends the cease-fire was broken by Tuesday's bus bombing in Jerusalem that killed 20 people. Both Hamas and Islamic Jihad claimed responsibility for that attack.

By late Thursday, four Qassam rockets were fired from Gaza into southern Israel as it appeared Hamas was acting on its word, witnesses said. No major damage was reported in Israel.

Witnesses said Israeli tanks moved into Beit Hanoun soon afterward in search of the perpetrators. In the West Bank, witnesses reported movement of Israeli tanks, coming from three directions.

With the situation rapidly deteriorating, the Bush administration's Mideast "road map" was facing one of its toughest challenges to date. At the United Nations, U.S. Secretary of State Colin Powell urged Palestinian and Israeli leaders to stick to the road map.

The alternative, Powell said, would be "a cliff that both sides will fall off of."

Former U.S. Sen. George Mitchell, who spent years working on Mideast peace, said Palestinians and Israelis must use "patience and perseverance" and realize that "for both sides there is no military solution to this."

"There is no easy way," Mitchell told CNN.

'Everything is put on hold'

In the Thursday attack, three Israeli missiles hit a car carrying senior Hamas leader, Ismail Abu Shanab, 53, and two of his bodyguards as his car drove through the Rimal neighborhood of Gaza City. Abu Shanab was considered a moderate and was an architect of the cease-fire.

Palestinian Prime Minister Mahmoud Abbas called the Israeli airstrike an "ugly crime" that was "against peace and the peace process."

Video from the scene showed a gutted car in flames with angry Palestinians gathering in the streets. Abu Shanab's body was carried to Shifa Hospital where thousands of Palestinians demonstrated, many of them calling on Abbas to resign.

Ismael Hania, an Hamas spokesman, addressed the crowd, vowing to avenge the death and calling on Abbas to resign.

"Leave the Palestinian territory immediately instead of fighting our people," Hania said of the new Palestinian prime minister.

According to an aide to Palestinian Authority security chief Mohammed Dahlan, the missile strike came shortly before Palestinian police intended to launch raids against Hamas and Islamic Jihad.

Elias Zananiri, Dahlan's spokesman, said Palestinian security forces had been given new rules of engagement, which would have allowed them to arrest militants and confiscate weapons. He said the new measures would have stripped Hamas and Islamic Jihad of their military wings.

"Now everything is put on hold," Zananiri said.

After Tuesday's bus bombing, Israel warned it cannot take further steps along the U.S.-backed road map to peace unless "substantial and meaningful" steps are taken against militants staging attacks.

Palestinian Authority Information Minister Nabil Amr called the Gaza attack "irresponsible" and said it would obstruct Palestinian efforts to crack down on militants.

"Now the Palestinian Authority has to re-evaluate the decisions from last night and the necessary steps taken to protect the Palestinian people," Amr said.